The present invention relates to a method for selectively recovering molybdenum and vanadium values from spent catalysts used for desulfurization of petroleums or, more particularly, relates to a method for selectively recovering simultaneously the molybdenum and vanadium values contained in spent catalysts used in the catalytic hydrogenation desulfurization of heavy oils by the technique of extraction.
As is well known, hydrogenation desulfurization of petroleums or, in particular, heavy oils is carried out usually by the catalytic reaction on a solid catalyst of molybdenum and cobalt supported on a carrier such as alumina. When the desulfurization reaction is run with a charge of the catalyst continuously over a long period of time, the catalyst is worn out and the catalytic activity thereof is decreased to such an extent that the reaction can proceed no longer at an industrially satisfactory velocity and with a desirable efficiency for desulfurization due to the deposition of heavy metals such as vanadium, nickel and the like as well as carbon, sulfur and the like contaminants originating in the heavy oil so that the catalyst thus worn out must be discharged out of the reactor as a spent catalyst and replaced with a new charge of the fresh catalyst.
It is highly desirable that the heavy metals and other components of value should be removed and recovered from the spent catalyst as completely as possible before the spent catalyst is discarded with objects not only to obtain economical advantages by the recovered values but also to prevent possible environmental pollution caused by the noxious or toxic ingredients such as the heavy metals contained in the spent catalyst.
Several methods are known for recovering heavy metals from such spent catalysts. These methods basically utilize the extraction of the heavy metal values with an aqueous acid or an alkali solution. Since the direct extraction treatment of the spent catalyst discharged out of the desulfurization reactor cannot give a satisfactory extraction of any one of the heavy metal values because these metal components are contained in the spent catalyst at least partly in the form of sulfides and/or lower oxides of the metals, it is conventional that the spent catalyst is calcined and oxidized at a relatively high temperature of 500.degree. C. or higher before extraction with an acid or alkali in order to convert all of the heavy metals content to the oxide forms readily decomposable and extractable by the acid or alkali. This process involving the oxidizing calcination of the spent catalyst is disadvantageous due to the necessary expensive facilities for the calcination and for the removal of noxious materials contained in the exhaust gas such as the sulfur dioxide and the smoke or soot produced by the combustion of the sulfur and carbonaceous or oily matters contained in the spent catalyst.
Alternatively, improved methods have been proposed for the extraction of the metal values from the spent catalyst without the step of preliminary calcination. In one of the methods, the spent catalyst is first treated with an aqueous caustic alkali solution to have the contents of molybdenum, vanadium and aluminum extracted into the alkali solution and then the residue of the above first extraction is treated with sulfuric acid to have the contents of nickel and cobalt extracted into the acid. In another of the methods, the spent catalyst is first subjected to leaching of nickel and cobalt in an oxidizing pressurized condition in the presence of water and the residue of the leaching is then subjected to the treatment with an alkali solution to have the contents of molybdenum and vanadium leached out into the alkali solution.
In the former method above, most part of the alumina component of the carrier is extracted by the alkali solution so that the subsequent process for the separation of molybdenum and vanadium from aluminum is necessarily complicated. In the extraction of the spent catalyst with water in the latter method, considerable amounts of the molybdenum and vanadium contents are extracted already in this first step so that difficulties are encountered in the clear-cut separation of these metals from nickel and cobalt. In addition, both of these methods are carried out at or above 120.degree. C. so that use of a pressurizable reaction vessel, i.e. autoclave, is indispensable.
In view of the above problems in the prior art methods, the inventor has undertaken investigations for developing an efficient method for the recovery of metal values from the spent catalyst used in the hydrogenation desulfurization of heavy oils with particular efforts to discover an extractant system with which the molybdenum and vanadium values can be selectively extracted with minimum extraction of the aluminum from the alumina carrier. The conclusion arrived at as a result of the investigations is that, among the alkaline extractants studied, i.e. sodium carbonate, ammonium carbonate, sodium hydroxide, ammonium hydroxide and others, best results are obtained with sodium carbonate in respect of the selectivity of the extraction of the molybdenum and vanadium contents.
Unfortunately, the use of a sodium carbonate solution alone as the extractant alkaline solution cannot give sufficiently high extraction of the molybdenum and vanadium contents. In an example, the use of an aqueous solution of a concentration of 160 g of Na.sub.2 CO.sub.3 per liter gave only 48.6%, 50.4%, 0.5%, 1.2% and 2.7% of extraction of the molybdenum, vanadium, nickel, cobalt and aluminum contents, respectively, in a spent catalyst.